A high-fidelity photon gun: intensity-squeezed light from a single molecule

A two-level atom cannot emit more than one photon at a time. As early as the 1980s, this quantum feature was identified as a gateway to "single-photon sources", where a regular excitation sequence would create a stream of light particles with photon number fluctuations below the shot noise. Such an intensity squeezed beam of light would be desirable for a range of applications such as quantum imaging, sensing, enhanced precision measurements and information processing. However, experimental realizations of these sources have been hindered by large losses caused by low photon collection efficiencies and photophysical shortcomings. By using a planar metallo-dielectric antenna applied to an organic molecule, we demonstrate the most regular stream of single photons reported to date. Measured intensity fluctuations reveal 2.2 dB squeezing limited by our detection efficiency, equivalent to 6.2 dB intensity squeezing right after the antenna.Comment: 9 pages, 3 figure

High-resolution spectroscopy of single Pr3+^{3+} ions on the 3^3H4_4-1^1D2_2 transition

Rare earth ions in crystals exhibit narrow spectral features and hyperfine-split ground states with exceptionally long coherence times. These features make them ideal platforms for quantum information processing in the solid state. Recently, we reported on the first high-resolution spectroscopy of single Pr$^{3+}$ ions in yttrium orthosilicate (YSO) nanocrystals. While in that work we examined the less explored $^3$H$_4$-$^3$P$_0$ transition at a wavelength of 488 nm, here we extend our investigations to the $^3$H$_4$-$^1$D$_2$ transition at 606 nm. In addition, we present measurements of the second-order autocorrelation function, fluorescence lifetime, and emission spectra of single ions as well as their polarization dependencies on both transitions; these data were not within the reach of the first experiments reported earlier. Furthermore, we show that by a proper choice of the crystallite, one can obtain narrower spectral lines and, thus, resolve the hyperfine levels of the excited state. We expect our results to make single-ion spectroscopy accessible to a larger scientific community.Comment: 5 pages, 5 figure

Few-photon coherent nonlinear optics with a single molecule

The pioneering experiments of linear spectroscopy were performed using flames in the 1800s, but nonlinear optical measurements had to wait until lasers became available in the twentieth century. Because the nonlinear cross section of materials is very small, usually macroscopic bulk samples and pulsed lasers are used. Numerous efforts have explored coherent nonlinear signal generation from individual nanoparticles or small atomic ensembles with millions of atoms. Experiments on a single semiconductor quantum dot have also been reported, albeit with a very small yield. Here, we report on coherent nonlinear spectroscopy of a single molecule under continuous-wave single-pass illumination, where efficient photon-molecule coupling in a tight focus allows switching of a laser beam by less than a handful of pump photons nearly resonant with the sharp molecular transition. Aside from their fundamental importance, our results emphasize the potential of organic molecules for applications such as quantum information processing, which require strong nonlinearities.Comment: 6 pages, 5 figure

Coherent Interaction of Light and Single Molecules in a Dielectric Nanoguide

We present a new scheme for performing optical spectroscopy on single molecules. A glass capillary with a diameter of 600 nm filled with an organic crystal tightly guides the excitation light and provides a maximum spontaneous emission coupling factor ($\beta$) of 18% for the dye molecules doped in the organic crystal. Combination of extinction, fluorescence excitation and resonance fluorescence spectroscopy with microscopy provides high-resolution spatio-spectral access to a very large number of single molecules in a linear geometry. We discuss strategies for exploring a range of quantum optical phenomena, including coherent cooperative interactions in a mesoscopic ensemble of molecules mediated by a single mode of propagating photons.Comment: 5 pages, 5 figure

Detection, spectroscopy and state preparation of a single praseodymium ion in a crystal

Solid-state emitters with atom-like optical and magnetic transitions are highly desirable for efficient and scalable quantum state engineering and information processing. Quantum dots, color centers and impurities embedded in inorganic hosts have attracted a great deal of attention in this context, but influences from the matrix continue to pose challenges on the degree of attainable coherence in each system. We report on a new solid-state platform based on the optical detection of single praseodymium ions via 4f intrashell transitions, which are well shielded from their surroundings. By combining cryogenic high-resolution laser spectroscopy with fluorescence microscopy, we were able to spectrally select and spatially resolve individual ions. In addition to elaborating on the essential experimental steps for achieving this long-sought goal, we demonstrate state preparation and read out of the three ground-state hyperfine levels, which are known to have lifetimes of the order of hundred seconds

A Sub-λ3\rm \lambda^{3}-Volume Cantilever-based Fabry-P\'erot Cavity

We report on the realization of an open plane-concave Fabry-P\'erot resonator with a mode volume below $\lambda^3$ at optical frequencies. We discuss some of the less common features of this new microcavity regime and show that the ultrasmall mode volume allows us to detect cavity resonance shifts induced by single nanoparticles even at quality factors as low as $100$. Being based on low-reflectivity micromirrors fabricated on a silicon cantilever, our experimental arrangement provides broadband operation, tunability of the cavity resonance, lateral scanning and promise for optomechanical studies

Coherent coupling of single molecules to on-chip ring resonators

We report on cryogenic coupling of organic molecules to ring microresonators obtained by looping sub-wavelength waveguides (nanoguides). We discuss fabrication and characterization of the chip-based nanophotonic elements which yield resonator finesse in the order of 20 when covered by molecular crystals. Our observed extinction dips from single molecules reach 22%, consistent with the expected Purcell enhancements up to 11 folds. Future efforts will aim at efficient coupling of a handful of molecules via their interaction with a ring microresonator mode, setting the ground for the realization of quantum optical cooperative effects

On-chip interference of scattering from two individual molecules in plastic

Integrated photonic circuits offer a promising route for studying coherent cooperative effects of a controlled collection of quantum emitters. However, spectral inhomogeneities, decoherence and material incompatibilities in the solid state make this a nontrivial task. Here, we demonstrate efficient coupling of a pair of organic molecules embedded in a plastic film to a TiO$_2$ microdisc resonator on a glass chip. Moreover, we tune the resonance frequencies of the molecules with respect to that of the microresonator by employing nanofabricated electrodes. For two molecules separated by a distance of about 8$\,\mu$m and an optical phase difference of about $\pi/2$, we report on a large collective extinction of the incident light in the forward direction and the destructive interference of its scattering in the backward direction. Our work sets the ground for the coherent coupling of several molecules via a common mode and the realization of polymer-based hybrid quantum photonic circuits

Spectral splitting of a stimulated Raman transition in a single molecule

The small cross section of Raman scattering poses a great challenge for its direct study at the single-molecule level. By exploiting the high Franck-Condon factor of a common-mode resonance, choosing a large vibrational frequency difference in electronic ground and excited states and operation at T < 2K, we succeed at driving a coherent stimulated Raman transition in individual molecules. We observe and model a spectral splitting that serves as a characteristic signature of the phenomenon at hand. Our study sets the ground for exploiting the intrinsic optomechanical degrees of freedom of molecules for applications in solid-state quantum optics and information processing.Comment: 7 pages, 5 figure